Clutch control system

ABSTRACT

A clutch control system having an engine speed sensor (26) for monitoring the speed of an engine (10), a clutch actuation sensor (34) for monitoring the operation of a clutch slave cylinder (22), a throttle position sensor (30) and a control unit (36) receives signals from the sensors (26, 34, 30) and determines when clutch control is required. When it is, the control unit switches to a reference mode in which, in a continuous closed loop operation, an engine speed signal Ve is compared to a reference speed signal Vr and the clutch is engaged or disengaged slightly to make the engine speed approach the reference speed.

This invention is concerned with the control of the clutch in a vehiclewhich has a semi automatic or fully automatic transmission.

Patents GB 2 088 007, and GB 2 079 888 describe means for controlling avehicle clutch when taking up from rest which use the concept of aclosed loop control whereby the engine speed is controlled so as toequal a pre-calculated reference speed by varying the degree ofengagement of the clutch. By this means it is possible to ensure thatthe vehicle moves off smoothly from rest, either gently or vigorouslyaccording to the drivers wishes, whether the vehicle is on the flat ormounting a hill.

Patent application WO 89/10282 describes a control system which includescontrol of the clutch during a gear change, and patent application WO89/03318 describes the control of the clutch during normal driving in amanner which eliminates driveline shock due to sudden changes ofthrottle. Both of these patent applications describe systems whichcalculate a degree of engagement of the clutch intended to give anoptimum slip torque, but due to inherent variability in the performanceof the friction material of the clutch facing the final effect may beslightly too harsh or too soft. It is the object of the presentinvention to extend the concept of the closed loop control of the firstmentioned patents to the control of the clutch in general driving innon-takeup gears to improve the smoothness of the ride during gearchanges, sudden throttle manipulations and other disturbances.

Other systems are known in which the engine speed is compared with thedriven plate speed directly, and which then attempt to set thisdifference at a finite value such as 150 rpm. It is a further advantageof the system here described that by comparing engine speed with acorrectly chosen reference the difference should be close to zero, whichmeans that the scaling of the parameter representing the difference canbe more advantageously chosen, enabling the control system to work withgreater accuracy. If the system is implemented in a microprocessor thecalculations within the speed loop need to be repeated at a greaterfrequency than other calculations. The design of the loop so that erroris near zero without further calculations is of assistance in minimisingthe calculation that has to be done within the loop.

The present invention provides a clutch control system for a vehiclehaving a clutch actuation means, a transmission and an engine, thesystem comprising sensing means including an engine speed sensor forproducing an engine speed signal variable with the speed of the engineand a clutch position sensor for producing a clutch position signalvariable with the state of engagement of the clutch; reference signalgenerating means for generating a reference signal; and a control meansoperable in a first mode wherein the control means compares thereference signal with the engine speed signal and operates the clutchactuation means so that the engine speed signal tends to approach thereference signal, and a second mode wherein the clutch actuation meansis not controlled by a reference signal, the control means beingarranged to switch between the two modes in response to signals from thesensing means.

Preferred embodiments of the present invention will now be described byway of example only with reference to the accompanying drawings inwhich:

FIG. 1 is a diagrammatic representation of a clutch control systemaccording to the invention;

FIG. 2 is a graph showing variation in time of various parameters withinthe system of FIG. 1 during a gear change;

FIG. 3 is a graph showing variation in time of various parameters withinthe system of FIG. 1 following a sudden demand for torque by a driver;

FIGS. 4a and 4b are graphs showing variation in time of variousparameters within the system of FIG. 1 when the driven plate speedexceeds a maximum safe engine speed; and

FIG. 5 is a graph showing variation in time of various parameters withinthe system of FIG. 1 when the engine speed falls below a set value.

Referring to FIG. 1 a vehicle clutch system comprises an engine 10 and agearbox 12 coupled through a friction clutch 14, via a gearbox inputshaft 15. Fuel is supplied to the engine by a throttle 16 which includesa throttle valve 18 operated by accelerator pedal 19. The clutch 14 isactuated by a release fork 20 which is operated by a slave cylinder 22.The gearbox is operated by a gear lever 24. A sensing means comprisingan engine speed signal generator comprising an engine speed sensor 26which includes a transducer and produces a signal Ve proportional toengine speed; a throttle valve position sensor 30 which produces asignal Vt proportional to the amount which the throttle is open; a pedalposition sensor 39 or torque demand sensor for producing a signalvariable with the position of the accelerator pedal 19 i.e., the demandfor torque from a driver; a gear position sensor 32 which produces asignal Vg corresponding to the gear which is engaged; a knobswitch 33 onthe gear lever 24 which detects forces applied to the gear lever 24 bythe driver and produces a signal Vs indicating an intention to changegear; an actuation sensor 34 which produces a signal Vc varying with theposition of the slave cylinder; and a driven plate speed sensor 42 whichproduces a signal Vdp proportional to the speed of the gearbox input,shaft 15, which is equal to that of a driven plate 40 of the clutch 14.Since the speed of the vehicle depends on the driven plate speed and thegear engaged, the driven plate speed sensor 42 acts in effect as avehicle speed sensor. Signals from the sensors are transmitted to acontrol unit 36 which controls the actuation of the clutch 14 via ahydraulic control 38. The control unit 36 includes a reference signalgenerator for generating a reference signal Vr. The throttle signal Vewhose idealised profile is shown in FIG. 2 is typical for a driveroperated throttle. This signal can be utilised if desired for theoperation of an automatic throttle control 37 operated by the controlunit 36. The throttle control 37 is operated by the control unit 36 sothat it can open or close the throttle independently of the acceleratorpedal 19.

Referring to FIG. 2, while the vehicle is driving for example in secondgear and accelerating, the control system is in a normal or drive modein which no clutch movement is required. At time t=0 the driver movesthe knobswitch 33 indicating that he intends to change to third gear.The control system then switches to an automatic mode. Control of thethrottle is taken from the driver and the throttle is closed by thethrottle control 37 in two stages. The clutch is disengaged by thehydraulic control 38 between the two stages of throttle closing. Also asthe throttle is closed the engine speed levels off gradually. At time t1second gear is disengaged. Thereafter the driven plate speed isdecreased rapidly by synchromesh and the engine speed decreasesgradually. At time t2 third gear is engaged and clutch engagementbegins. The clutch is engaged at a set rate until it reaches the minimumengagement limit or kiss point at t3. The control system then switchesto a reference mode, with the reference signal Vr being set to aninitial equal to the engine speed signal Ve. The reference signal Vrthen follows a curve of a predetermined general shape, falling offduring initial clutch engagement and then rising as clutch engagement iscompleted. The engine speed follows the reference speed as describedbelow. The exact shape of the Vr curve is dependent on how depressed theaccelerator pedal 19 is so that if the driver wishes to acceleratequickly, the clutch will engage quickie. After a delay from engagementof third gear, the throttle valve 18 is opened at a steady rate until att4 its position corresponds to the position of the accelerator pedal 19.Then control of the throttle is returned to the driver. At time t5 whenthe engine speed and driven plate speed are equal the control loop isabandoned and the clutch is engaged at a fixed gradual rate to fullengagement to t6. The control system then switches to the normal ordrive mode, awaiting another signal indicating that clutch control isagain required.

In the reference mode the clutch engagement is governed by the controlunit 36 so as to keep engine speed signal Ve equal to the referencesignal Vt. This is done by a closed loop operation in which the enginespeed signal Ve is compared with the reference signal Vr and an errorsignal Vs is produced which can be either positive or negative. Theclutch is then engaged or disengaged to make the error signal tend tozero. That operation is continuously repeated. In this case because theengine is driving the driven plate, if Ve is greater the Vr, i.e., Vs isnegative, the clutch is engaged slightly to slow down the engine. If Vris greater than Ve, the clutch is disengaged to allow the engine tospeed up.

The above operation for an up-change in gear ratio can be used inprinciple for a down-change in gear ratio. Referring to FIG. 3, when, attime t=0 the throttle signal Vt falls below a set value Vn, the clutchis partially disengaged, and Vc decreases so that its torquetransmitting capacity is reduced. While the throttle signal remains lowor increases gradually, no clutch slip occurs. However if, at t1, theaccelerator pedal is depressed suddenly, by more than a set amountincreasing the throttle signal, the control system switches to referencemode and a reference signal Vr is set at an initial value which isgreater than the driven plate signal Vdp. This will cause the clutch todisengage and Vc to decrease and allow clutch slip so that the enginecan speed up towards the reference value. That slip prevents the suddenthrottle opening from producing a torque surge in the drive line. Thereference signal Vr is then set to follow a precalculated profilereturning to the driven plate speed after a short period, typically 400ms. This causes the clutch to return to near full engagement in thatperiod. When the engine speed and driven plate speed are nearly equal att3 the clutch is engaged at a steady rate until full engagement at t4.The conditions under which the clutch is initially partially disengagedmay include a requirement that a low gear, such as first or second gearis engaged.

In a further application of the invention as shown in FIG. 4 the controlsystem is used to prevent the engine from rotating above a maximum safespeed while retaining maximum possible engine braking. Between times t0and t1, the engine speed and driven plate speed are identical as thevehicle accelerates with the clutch fully engaged. Ve and Vdp are infact equal although they are separated in FIG. 4 for clarity. At time t1Vdp and Ve reach a set value Vem which corresponds to the maximum safeengine speed. This may have occured, for example if the vehicle brakeshave failed on a downhill gradient and a low gear has been engaged forbraking. The control unit therefore switches to the reference mode withthe reference signal set to equal to Vem. In this case, in the referencemode, because the driven plate is driving the engine, if Ve is greaterthan Vem, the clutch is disengaged slightly to allow the engine speed todecrease, and if Ve is less than Vem the clutch engages slightly toproduce further braking. When at time t2, the driven plate speed fallsbelow Vem the clutch will be fully engaged and the control signalswitches out of reference mode. During the time period when Vdp isgreater than the reference Vem, a warning device can operate to alertthe driver to the fact that the clutch should not engage the enginebecause the driven plate speed is greatly in excess of the engine speed.This could occur if the driver accidently engages 1st gear at high roadspeeds.

The control unit can also be used to reduce driveline vibration as shownin FIG. 5. When the engine is running slowly, irregularities in theengine rotation speed, caused for example by the firing of thecylinders, become noticeable. To prevent the irregularities from beingtransmitted to the gearbox the engine speed signal Ve is monitored andwhen it falls below a set value at t1, the control unit switches toreference mode, the reference signal being set to remain a predeterminedamount above Vdp. This keeps the clutch in a continuous state of slightslip and the irregularities in engine speed are not transmitted, or areonly partially transmitted, to the driven plate and gearbox.

I claim:
 1. A system for the control of a clutch in a vehicle having anengine which drives a multi-ratio transmission via the clutch, thesystem being arranged to control the clutch during ratio changes in thetransmission and other drive disturbances when the vehicle is in motionand comprising a clutch actuation means; sensing means for sensing aplurality of variable vehicle operating parameters including at leastengine speed, clutch engagement position, ratio engaged, and a desire tochange the operative ratio of transmission; reference signal generatingmeans for generating a reference signal; and a control means arranged todisengage the clutch on the occurence of a signal indicating a desire tochange the operative ratio of the transmission and to subsequentlyre-engage the clutch on selection of another ratio, the clutch beingre-engaged initially in a second mode of operation wherein the clutchactuation means is not controlled in accordance with the referencesignal and subsequently during each re-engagement in a first mode ofoperation in which the control means compares the reference signal withthe engine speed signal and operates the clutch actuation means to thatthe engine speed signal tends to approach the reference signal.
 2. Aclutch control system according to claim 1 wherein when the controlmeans switches to the first mode the reference signal is set to aninitial value dependent on the instantaneous value of a variable vehicleparameter.
 3. A clutch control system according to claim 2 wherein theinitial value of the reference signal is dependent on the instantaneousvalue of a plurality of variable vehicle parameters.
 4. A clutch controlsystem according to claim 1 wherein the reference signal varies in timewith the value of at least one variable vehicle parameter.
 5. A clutchcontrol system according to claim 1 wherein the reference signal variesin time according to a predetermined pattern.
 6. A clutch control systemaccording to claim 1 wherein the control means causes the clutch todisengage at a predetermined rate.
 7. A clutch control system accordingto claim 6 wherein the control means switches to the first mode when theclutch is re-engaged to its minimum engagement limit.
 8. A clutchcontrol system according to claim 1 further comprising a torque demandsignal generator for producing a torque demand signal variable with ademand for torque, the control means being arranged to switch to thefirst mode when the torque demand signal increases suddenly.
 9. A clutchcontrol system according to claim 8 wherein the initial value of thereference signal is a predetermined amount greater than the engine speedsignal.
 10. A clutch control system according to claim 1 wherein thecontrol means also switches to the first mode when the control meansdetects that the engine speed exceeds a set maximum value, the referencesignal representing a maximum permissible engine speed.
 11. A clutchcontrol system according to claim 1 wherein the control means alsoswitches to the first mode when the engine speed signal falls below apredetermined value.
 12. A system for the control of a clutch in avehicle having an engine which drives a multi-ratio transmission via theclutch, the system being arranged to control the clutch during drivedisturbances when the vehicle is in motion and comprising a clutchactuation means; sensing means for providing signals indicative of aplurality of variable vehicle operating parameters including at leastengine speed, clutch engagement position and torque demand; referencesignal generating means for generating a reference signal; and a controlmeans operable in a first mode wherein the control means compares thereference signal with the engine speed signal and operates the clutchactuation means so that the engine speed signal tends to approach thereference signal, and a second mode wherein the clutch actuation meansis not controlled by a reference signal, the control means beingarranged to switch to the first mode when the torque demand signalincreases suddenly.
 13. A system for the control of a clutch in avehicle having an engine which drives a multi-ratio transmission via theclutch, the system being arranged to control the clutch during drivedisturbances when the vehicle is in motion and comprising a clutchactuation means; sensing means for providing signals indicative of aplurality of variable vehicle operating parameters including at leastengine speed and clutch engagement position; reference signal generatingmeans for generating a reference signal; and a control means operable ina first mode wherein the control means compares the reference signalwith the engine-speed signal and operates the clutch actuation means sothat the engine speed signal tends to approach the reference signal, anda second mode wherein the clutch actuation means is not controlled by areference signal, the control means being arranged to switch to thefirst mode when the engine speed exceeds a set maximum value.
 14. Asystem for the control of a clutch in a vehicle having an engine whichdrives a multi-ratio transmission via the clutch, the system beingarranged to control the clutch during drive disturbances when thevehicle is in motion and comprising a clutch actuation means; sensingmeans for signals indicative of a plurality of variable vehicleoperating parameters including at least engine speed and clutchengagement position; reference signal generating means for generating areference signal; and a control means operable in a first mode whereinthe control means compares the reference signal with the engine speedsignal and operates the clutch actuation means so that the engine speedsignal tends to approach the reference signal, and a second mode whereinthe clutch actuation means is not controlled by a reference signal, thecontrol means being arranged to switch to the first mode when the enginespeed falls below a predetermined value.